1. Field of the Invention
This invention relates to a voltage supply circuit used in a semiconductor integrated circuit such as a memory device, etc., and more particularly to a voltage supply circuit for operating an internal circuit by using a predetermined reference voltage.
2. Description of the Prior Art
In MOS type semiconductor integrated circuit devices, a technology for allowing an internal power supply voltage to be down converted has been used from a viewpoint of device reliability, etc. (See, e.g., Nikkei Microdevice, No. 52, pp. 91 to 95, October, 1989).
Meanwhile, in semiconductor memories such as a static RAM, etc., there is a mode in a standby state for interrupting the circuit to suppress current consumption (standby mode) in addition to a mode in an operating state for performing an ordinary read/write operation. It is necessary to suppress a current consumption in the standby state.
FIGS. 1 to 3 show conventional internal voltage supply circuits, respectively.
First, in the circuit shown in FIG. 1, an external power supply voltage Ext-Vcc is delivered from an external terminal 5 to a reference voltage generator 1 for generating a predetermined internally down converted reference voltage. An output from the reference voltage generator 1 is inputted to the minus terminal of a differential amplifier 3. An output from the differential amplifier 3 is delivered to the gate of a pMOS transistor 4. The drain of the pMOS transistor 4 is connected to the plus terminal of the differential amplifier 3, and a potential on the drain and an output voltage of the reference voltage generator 1 are imaginarily shorted. The potential on the drain of the pMOS transistor 4 is delivered to the internal circuit as an internal power supply voltage Int-Vcc. Further, a standby power supply voltage converter 2 of a low power consumption is provided independently of the reference voltage generator 1. In use, switching between the reference voltage generator 1 and the standby power supply voltage converter 2 is carried out in response to a chip enable signal CE.
Further, the circuit shown in FIG. 2 includes a reference voltage generator 11, a differential amplifier 13 and a pMOS transistor 14 which are similar to those of the circuit shown in FIG. 1, respectively. The feature of the FIG. 2 circuit is as follows. The standby power supply voltage converter 12 is operating at all times. In response to a chip enable signal CE, the reference voltage generator 11 and the differential amplifier 13 are controlled so that they are operative. The reference voltage generator 11 and the differential amplifier 13 are placed in an OFF state in the standby state.
In addition, the circuit shown in FIG. 3 includes a single reference voltage generator 21 for generating a reference voltage, and two differential amplifiers, i.e., a differential amplifier 22 for use in the operating state and a differential amplifier 23 for use in the standby state. In this circuit, the reference voltage generator 21 is operating at all times. In the operating state, the differential amplifier 23 is placed in an OFF state. Outputs from the differential amplifiers 22 and 23 are inputted to the pMOS transistors 24 and 25, respectively. Thus, an internal power supply voltage Int-Vcc is taken out from the drain commonly connected of the pMOS transistors 24 and 25.
However, with the above-described circuits (FIGS. 1 to 3), the following problem would occur.
First, in the circuit shown in FIG. 1 or 2, for a transient time period from the standby state to the operating state, it is necessary to wait rising of outputs from the reference voltage generator 1 or 11, and/or the differential amplifier 3 or 13 which have been respectively in OFF state. For example, if these circuits have no setup time, the internal power supply voltage Int-Vcc disadvantageously varies, or the like. In contrast, if these circuits have a setup time, the time required for CE access is delayed by that time.
Further, in the case of the circuit shown in FIG. 3, at the time of switching from the standby state to the operating state, the differential amplifier 23 is only switched to the differential amplifier 22. The setup time can be reduced to a relatively short time required for rising of an output from the differential amplifier 22. However, since the reference voltage generator 21 operates at all times, and the differential amplifier 23 is operating even in the standby state, the power consumption becomes large.